Generally we use "tight" and "loose" when referring to our belts.
Examples:
"My belt is too tight. I can hardly breathe."
"My belt is too loose and my pants are falling down a bit."
As for the holes in a belt...I'd probably just say "belt holes." If there is some other fancy term, I'm unaware of it.
Incidentally, the loops on your pants through which you put your belt are called "belt loops."